Thermoplastic prealloyed powder

ABSTRACT

Prealloyed powders, particularly of compositions which are difficult to work by conventional means, are subjected to forces such that strain energy is imparted thereto by virtue of which the prealloyed powders manifest low flow stress and a high degree of thermoplasticity.

United States Patent 1 Volin et al.

[451 Feb. 11, 1975 I THERMOPLASTIC PREALLOYED POWDER [75] Inventors: Timothy Earl Volin, Tuxedo; John Stanwood Benjamin, Suffern; Jay Michael Larson, Warwick; Robert Lacock Cairns, Suffern, all of NY.

[73] Assignee: The International Nickel Company,

Inc., New York, NY.

[22] Filed: Dec. 18, 1972 [21] Appl. No.: 316,077

[52] U.S. Cl. 75/.5 R, 75/.5 BA, 75/.5 BB,

75/.5 BC [51] Int. Cl B22f 9/00 [58] Field of Search 75/.5 BA, .5 BB, .5 BC.

75/.5 AA. .5 AB, .5 AC, 171, 176, 128 R. 128 F. 128 (3,128 Z. 128 T, 128 W, 128 V: 148/115 F. 126; 29/1825 [56] References Cited UNITED STATES PATENTS 3.519.503 7/1970 Moore et a1. 148/1 1.5 F 3.591.362 7/1970 Benjamin 75/.5 BA 3.631.583 l/1972 Haller 29/4205 3.639.179 2/1972 Reichmzm et a1. 148/115 F 3.671.230 6/1972 Smythe et a1. 75/213 3.728.088 4/1973 Benjamin 29/1825 FOREIGN PATENTS OR APPLICATIONS 1,298,944 12/1972 Great Britain OTHER PUBLICATIONS Reichman et al., Developments in Superalloy Powders," Modern Developments in Powder Metallurgy, Vol. 5, pp. 73-84.

Reinhardt et a1., Cold Consolidation of Metal Plus Dispersoid Blends for Examination by Eleotron Microscopy, NASA TN D-351 1, July 1966.

Freche et al., Evaluation of Two Nickel-Base Alloys, Alloy 713C and NASA TAZ-8A, Produced by Extrusion of Prealloyed Powders, NASA TN D-5248. May 1969.

Primary E.\'aminer-W. Stallard Attorney, Agent, or Firm-Ewan C. MacQueen; Raymond J. Kenny [57] ABSTRACT Prealloyed powders, particularly of compositions which are difficult to work by conventional means, are subjected to forces such that strain energy is imparted thereto by virtue of which the prealloyed powders manifest low flow stress and a high degree of thermoplasticity.

4 Claims, 1 Drawing Figure Ew cyfirugg T 1 THERMOPLASTIC PREALLOYED POWDER During recent years, research efforts have been intensive in the quest for new alloys capable of meeting the stringent requirements imposed by more severe operating conditions, notably higher temperatures. And this has been particularly evident in respect of alloy development involving gas turbine engines.

To withstand the destructive effect of elevated temperature has of necessity required alloys of greater strength and hardness. But the very improvements achieved with regard to such metallurgical properties have, in turn, been responsible for placing an increasingly heavier burden upon hot workability and fabricability characteristics in general. This rather ironic de' velopment has reached the point at which the most desirable alloys, e.g., the alloy known as IN-l00, virtually cannot, as a practical matter, be conventionally hot worked. As a consequence, such materials have been largely produced in cast form only. Suffice to say, casting is inherently self-limiting.

To produce the exceptionally difficultly workable alloys, the metallurgist has turned to such techniques as powder metallurgy. In this connection, the recently introduced gatorizing concept has been utilized and has proved to be of considerable merit. This advance appears, however, to suffer from the drawback in that the required extrusion reduction places a limitation on the section size of products that can be made with available equipment. This does focus attention on an existing problem, to wit, product shapes, and presages other difficulties arising.

In the near future it is expected that huge industrial or power turbines will be coming on stream and these may bring forth operating or processing characteristics even more demanding in certain respects than present turbine engines. For example, discs of 4 to 5 feet or more in diameter loom as a distinct probability. Yet, given the high strength requirements needed for elevated temperature operation, equipment is not presently available domestically, insofar as we are aware,

having the capacity to deliver the forces necessary to satisfactorily press a compact, of say, IN-- 100, to such size. And as is known, the capital investment involved in this type of equipment is of considerable magnitude.

However, in accordance with the present invention, it is deemed that extremely large diameter discs (as well as much smaller turbine discs) and a host of other components can be produced without recourse to new and costly equipment. This stems from the discovery, as will be more fully described herein, that if high strain energy is induced into powder prealloyed to desired composition, e.g., virtually unworkable INl0O, the nature of the powder is altered to such an extent that the powder takes on the physical characteristic of thermoplasticity, exhibiting significantly less resistance to flow. As a direct consequence of this phenomenon, (a) lower pressing temperatures and/or forming stresses can be used than otherwise would be the case, (b) the capacity of available equipment would be capable of producing huge shapes such as 4-5 foot diameter turbine discs, (0) processed powder can be put into a preform closely corresponding to the configuration of shape of the final product, (d) the preform is capable of retaining its relative ease of deformability properties through subsequent processing, and (e) limitations introduced by being otherwise restricted in the type of apparatus that can be used, e.g., the extrusion press, are greatly minimized if not obviated.

As will be appreciated by those skilled in the art, the amount of strain energy required will be influenced by the hardness and strength of a given alloy at a given elevated temperature. And this to a considerable extent is largely a reflection of composition. To indicate generally when sufficient strain energy has been imparted to prealloyed powder to confer thermoplasticity" as contemplated herein, reference is made to the accompanying drawing.

Curve A represents prealloyed powder which has been subjected to strain energy, Curve B representing prealloyed powder of the same composition but which has not been so processed. Point I-I represents a common hardness value for each of the prealloyed powders at a given temperature, the respective powders having been consolidated to a density of at least 99 percent of theoretical.

In accordance herewith, if an amount of strain energy has been imparted to prealloyed powder such that at the point /2 H AT/TM (the temperature differential, A T, between the respective hardness curves divided by the absolute melting temperature of the alloy, TM) is at least 1 percent, the prealloyed powder is deemed thermoplastic. However, this thermoplastic condition, referred to as TPC1 (Thermoplastic Physical Characteristic), is considered to be minimal. Preferably this ratio (AT/TM) should be at least 2 percent (TPC-Z) and most advantageously at least about 5 percent (TPC-3). This contributes greatly to minimum flow stress and lower pressing temperatures which in turn reduce the otherwise required load on a press (or equivalent functioning equipment).

It is conceivable that some materials may not show an H, hardness value. This could be the case in respect of, for example, a material in which the increase in hardness due to the strain energy input is less than that of a hardening phase destroyed during the energy input. Too, it is considered that there are alloy materials in which an H value exists at a lower temperature than the lower limit hardness test temperature. In such instances, the H value would be replaced by the expres- Sion l( .4 )RT/2] ahn/ (HA)RT being the room temperature hardness of the prealloyed powder and (H being the room temperature hardness of the same powder in the processed condition. It is to be understood that the claims appended hereto are to be so construed with regard to Thermoplastic Physical Characteristic values. Thus, at k [(H /2 (H h /2], the AT/TM ratio must be at least 1 percent in order for the processed powder to be considered thermoplastic.

It has been found that the necessary strain induced energy can be readily imparted to the prealloyed powders by employing a technique similar to the mechanical alloying process as described in US. Pat. No. 3,591,362 and incorporated herein by reference. This is a dry, intensive high energy milling process in which constituent powders are subjected to continuous and repeated compressive impact forces, for example, by melting media, until a composite product alloy powder of substantial saturation hardness and of a composition to the respective percentages of the initial individual constituent powders is produced. The constituent powders become most intimately interdispersed, the alloy being exceptionally dense and homogeneous. In the subject invention, attaining saturation hardness is not necessary (in certain instances it may be undesirable) and, surprisingly, though prealloyed powder is used it responds exceptionally well to strain energy being imparted thereto at room temperature.

As to the milling process, the required compressive impact forces may be applied, for example, by milling elements, normally balls formed of steel, nickel, tungsten carbide, etc., which are kinetically maintained in a high state of relative motion, at least 50 percent to 75 percent of the balls being preferably maintained out of static self-contact at any given time. This assures that the accelerative forces will cause the ball elements to continuously and repeatedly collide. Spex mills, attritor mills such as the Szegvari attritor mill, vibratory mills and planetary ball mills can be used to induce the necessa'ry strain energy. Conventional ball mills, as a general proposition, even if they can supply the necessary energy, require too long a period to do so. This is illustrated herein in connection with the data presented in Table IV.

Of course, the important consideration is not the particular type of machine utilized but rather the degree of strain energy input to the prealloyed powders. To that end, the information given in Table 1 below is intended to indicate simply by way of illustration only, satisfactory processing parameters for a machine such as an attritor in which 5/16 dia. 52100 through hardened balls are used in conjunction with an argon percent methane atmosphere (other atmospheres e.g.,

nitrogen or nitrogen-oxygen atmospheres, can be used.

TABLE I Tank Impeller Ball Powder Time, Apparatus diam. speed, rpm ratio (vol.) hr.

1 gal. 9 300-400 5-50 l-lO attritor 4 gal. 13 200-300 do. l-l0 attritor 10 gal. 16 125-200 do. l-10 attritor 100 gal. 36 50-150 do. l-lO attritor With regard to powder size, a broad range may be used, e.g. up to 1000 microns; it is preferred, however, in using 5/16 inch diameter 52100 balls to use powder within the range of 20 to 350 microns.

To give those skilled in the art a better appreciation of the invention, the following Examples and data are given:

EXAMPLE I Atomized IN-l00 powder containing (nominally).

about 15% Co, 10% Cr, 5.5% A1, 4.75% Ti, 3% Mo, 0.02% C, 0.15% B, 0.06% Zr, balance essentially Ni, was screened to minus 325 mesh. It was then subjected to high energy impact in 1 gallon attritor for a period of about 10 hours, the impeller speed being maintained at about 350 rpm, 37 pounds of through hardened 52100 steel balls (5/16" dia.) being used as the impacting elements. The ball to powder (volume) ratio was about 17:1 and a flowing nitrogen atmosphere'was used.

The as-attrited powder was then canned in mild steel. evacuated, heated to 600F. under vacuum and sealed.

Compaction was accomplished by heating the sealed can to 1900F. and then upsetting the same against a blank die in an extrusion press. This was followed by vermiculite cooling. l-lot hardness was determined on the as-compacted material as shown in Table 11. Included for purposes of comparison are the corresponding hardness values of the lN100 powder in the asatomiz ed and compacted state.

RT room temperature The foregoing reflects the tremendous drop in hardness as the temperature exceeded about 1400F. At 1600F., hardness was already at the dramatically low level of 14.4 At 1800F. the hardness of the strain energy induced (attrited) powder was at such a low level that it could not be determined on the Rockwell A scale. In marked contrast, it will be observed that the hardness oflNl00 in the as-atomized and compacted condition was, comparatively speaking, still quite high at the corresponding temperatures. At the temperature 1600F., for example, the attrited powder in comparison with its as-atomized counterpart had a Thermoplastic Physical Characteristic TPC3.

EXAMPLE 11 Another batch of minus 325 mesh atomized lN-100 powder was processed as in Example I. In this instance, compacting in the extrusion press was conducted at 1950F. Tensile specimens were prepared from the processed billet. One specimen was tested in the attrited and as-compacted state at 1800F. Flow stresses were determined and were found to range from 4870 to 11,170 psi over strain rates of from 0.0025 to 0.625 min. At fracture, the elongation was 144 percent and the reduction in area was 99 percent. In marked contrast the flow stress for as-atomized lN-l00 upset compacted at 1950F. was from 8670 to 16,000 psi over the same strain rates, the elongation being 10 percent and reduction in area being 4.5 percent.

The rather striking differences in flow stress and/or ductility between the attrited and as-atomized powders are deemed attributable, at least in part, to the much finer grain size or dislocation structure of the attrited powder. Moreover, it is considered that there is a finer dispersion of embrittling phases as a by-product of the attriting. Compaction of the as-atomized powder is rendered most difficult at best by reason of phases on the powder surface and by the higher deforming stresses required. This contributes to subsequent problems concerning porosity and embrittlement.

For purposes of determining stress-rupture life, another tensile specimen from the attrited compacted powder was heat treated at 2100F. for 4 hours, oil

quenched, heated at I200F. for 24 hours, air cooled, and aged at l400F. for 8 hours followed by air cooling. At a temperature of I350F. and under a stress of 100,000 psi the specimen had a rupture life of 22.5

relationship to the matrix structures surrounding them, it would be at random). ln our thermoplastic prealloyed powder in the consolidated form much ofthc carbide is present in the form of finely dispersed scmihours. This compares with but 2 hours for the as 5 coherent particles. The reason for the difference in the atomized compacted powder treated in the same fashcarbide structure can be attributed to the great amount io In the latter 0388, Strength Could o be imPFOVCd of deformation induced in the powder particles to inby ea e ment troduce the high strain energy state. The pre-existing carbides in the powder are comminuted and redistrib- EXAMPLE m uted by this deformation process. Upon subsequent Batches of atomized lN- l00 powder of a composiheating to elevated temperature of compaction it betion corresponding to that given in Example I, screened comes energetically favorable from a thermodynamic to minus 60 plus 325 mesh were processed in a 5 /2" diviewpoint for the carbides to dissolve and reprecipitate. ameter ball mill. The ball mill was run at about 80 This occurs at temperatures where atomic mobility is rpm using 3/8" diameter, through hardened 52100 relatively low and chemical potential is high. This leads steel balls, a 10:1 ball to powder ratio and a sealed air to the formation ofa uniquely fine, coherently precipiatmosphere, the difference being that in one case the tated carbide structure. In the conventional prealloyed process was conducted for 5 hours as opposed to 50 powder the carbides are not comminuted and are hours in the second instance. Two other batches of therefore retained in a relatively stable, large blocky minus 325 mesh atomized lN-100 powder were atform. Consolidation and working of conventional trited as in Example I for periods of l and 3 hours, reprealloyed powders must be done at relatively higher spectively. Each of the 4 batches was compacted as in temperatures. Some solution of the large, blocky car- Example II and hardness determinations were made at bide occurs. The higher temperatures also increase various temperatures as given in Table III. Also sct atomic mobility, so that what carbide does dissolve is forth, for convenience, are the hardness values for asmore likely to segregate to grain boundaries or other atomized and compacted (no strain induced energy) surfaces upon cooling. powder and the IN-I00 atomized and attrited powder By virtue of the thermoplastic nature conferred upon of Example I (attrited 10 hrs.). Finally, hardness data the prealloyed powders as a result of the invention, :1 taken from that published for conventional as-cast variety of consolidation techniques can be employed, lN I00 is also included. including press forging, creep forging, vacuum hot TABLE IV Rockwell Hardness, RH,

Temp. As- Ball Milled Attrited As F. Atomized 5 Hr. Hr. l H1. 3 Hr. 10 Hr. Cast too low to ineasun- RT. room temperature The above results show that the attriting for but 1 50 pressing, forging and rolling, hot isostatic compaction hour rendered the lNIOO powder vastly more therat temperatures lower than what otherwise might be moplastic than dry ball milling for 5 hours and somethe case, upsetting on an extrusion press, etc. In this what more deformable than ball milling for the less connection, the smaller turbine discs can be closed die practical period of 50 hours, this being a direct effect press forged with shaped rams. The as-formed disc of the amount of strain energy induced in the respecwould be virtually completely porosity free and ready tive powders. As a practical matter, the as-cast hardfor heat treatment and machining. For large discs the ness remained rather unchanged over the complete ultimately formed billet can be cross rolled or closed temperature range of test. die press forged to near final shape and size. A most at- In addition to the foregoing, our thermoplastic prealtractive feature in either case is the minimum of waste, loyed powder is considered to be significantly morphothe superalloys being quite costly, coupled with a minilogically or structurally different from conventional mum of finish machining. prealloyed powder in the final consolidated form. This In the above describing the instant invention, the is particularly true in respect of the so-termed superalcommercially well known alloy IN- 100 has been used. loys which always contain carbon. In the conventional The invention is obviously not restricted to this particuprealloyed powder the carbides generally tend to be relatively large, e.g., greater than I micron, and are incoherent in the matrix (atomic structure would have no lar composition. To be sure, the invention is primarily directed to difficulty workable superalloys but it is applicable to alloys in general. Among the superalloys are 7 those containing up to 60%, e.g., l to 25%, chromium; up to 30%, e.g., to 25%, cobalt; up to e.g., l to 9%, aluminum; up to 8%, e.g., l to 7%, titanium, and particularly those alloys containing 4 or 5% or more of aluminum plus titanium; up to 30%, e.g., l to 8%, molybdenum; up to 25% e.g., 2 to tungsten; up to 10% columbium; up to 10% tantalum; up to 7% zirconium; up to 0.5% boron; up to 5% hafnium; up to 2% vanadium; up to 6% copper; up to 5% manganese; up to 70% iron; up to 4% silicon, and the balance essentially nickel. Cobalt-base alloys of similar composition can be treated. Among the specific superalloys might be listed IN738 and 792, Rene alloys 41 and 95, Alloy 718, Waspaloy, Astrology, MarM alloys 200 and 246, Alloy 713, Alloys 500 and 700, A-286, etc. Various of these alloys are more workable than others. Other base alloys such as titanium can be processed as well as refractory alloys such as SU16, TZM, Zir-' caloy, etc. Prealloys contemplated herein can contain up to 10 percent or more by volume of a dispersoid such as Y O T150 La O etc.

Finally, it will be understood that modifications and variations of the invention may be resorted to without departing from the spirit and scope thereof as those skilled in the art will readily understand. Such are considered to be within the purview and scope of the ins vention and appended claims.

We claim: 1,. New and improved superalloy prealloy powder,

. said powder containing up to 60% chromium, up to 30% cobalt, up to 8% titanium, up to 30% molybdenum, up to 25% tungsten, up to 10% columbium, up to 10% tantalum, up to 7% zirconium, up to 0.5% boron, up to 5% hafnium, up to 6% copper, up to 5% manganese, up to iron, up to 4% silicon, and the balance essentially nickel, and being further characterized in that it is in the strain energy induced condition with a Thermoplastic Physical Characteristic of at least TPC-l.

2. New and improved superalloy prealloy powder in accordance with claim 1 having a Thermoplastic Physical Characteristic of at least TPC-2.

3. New and improved superalloy prealloy powder in accordance with claim 1 having a Thermoplastic Physical Characteristic of at least TPC-3. I

4. New and improved superalloy prealloy powder in accordance with claim 2 in which the prealloy is a member of the group consisting of IN- 100, lN738, lN--792, Rene alloys 41 and 95, Waspaloy, Astroloy, Alloys 500, 700, 713 and 718, MarM alloys 200 and 246 and Alloy A286. 

1. NEW AN IMPROVED SUPERALLOY PREALLOY POWDER, SAID POWDER CONTAINING UP TO 60% CHROMIUM, UP TO 30% COBALT, UP TO 8% TITANIUM, UP TO 30% MOLYBDENUM, UP TO 25% TUNGSTEN, UP TO 10% COLUMBIUM, UP TO 10% TANTALUM, UP TO 7% ZIRCONIUM, UP TO 0.5% BORON, UP TO 5% HAFNIUM, UP TO 6% COPPER, UP TO 5% MANGANESE, UP TO 70% IRON, UP TO 4% SILICON, AND THE BALANCE ESSENTIALLY NICKEL, AND BEING FURTHER CHARACTERIZED IN THAT IT IS IN THE STRAIN ENERGY INDUCED CONDITION WITH A THERMOPLASTIC PHYSICAL CHARACTERISTIC OF AT LEAST TPC-1.
 2. New and improved superalloy prealloy powder in accordance with claim 1 having a Thermoplastic Physical Characteristic of at least TPC-2.
 3. New and improved superalloy prealloy powder in accordance with claim 1 having a Thermoplastic PhYsical Characteristic of at least TPC-3.
 4. New and improved superalloy prealloy powder in accordance with claim 2 in which the prealloy is a member of the group consisting of IN-100, IN-738, IN-792, Rene alloys 41 and 95, Waspaloy, Astroloy, Alloys 500, 700, 713 and 718, Mar-M alloys 200 and 246 and Alloy A-286. 